1. Field of the Invention
This invention relates to data transmission in a wireless environment, and is specifically directed to a method for reliably passing modem handshaking information over a cellular radio link.
2. Summary of the Prior Art
With the introduction of cellular modem technology in the mid-1980""s, personal computing has reached a new level of mobility. No longer is the traveling business limited to PSTN (public switched telephone network) outlets or specialized private mobile links in order to access data remotely. Instead, the cellular modems permit wireless data exchange between field personnel and a home office over conventional cellular radio networks which increasingly span the globe.
Data on cellular is especially attractive because of the immense international cellular coverage area, the high penetration of mobile cellular handsets into the marketplace, the relatively modest RF power amplifier requirements making it well-suited for a mobile environment, and the capability to relay both voice and modulated data reasonably well in full duplex mode. Also, startup and subscription costs are much lower than current wireless alternatives which require specialized data-only mobile transceivers having limited geographical access.
In operation, the cellular modem, which is typically coupled between the laptop or mobile computer and the mobile handset, communicates with a land based PSTN modem or another cellular modem through traffic channels granted to the mobile handset for the duration of the call. Ideally, the mobile station should appear as just another PSTN modem to the remote land modem. However, unlike conventional fixed network telecommunications, cellular radio is an inherently xe2x80x9cdirtyxe2x80x9d data carrier, with perceived Bit Error Rates (BERs) approaching 2% (i.e., one error per 50 bits) at conservative 2400 baud transmission rates (by contrast, PSTN modems are unlikely to experience BERs exceeding 1xc3x9710xe2x88x925 at this speed). This is because, as a low power line-of-sight radio technology, cellular signals are susceptible to fading, shadowing and even dropout because of changing environmental conditions which momentarily block the transmission pathways. Noise and multipath interference also can severely degrade the signal. These events can result in signal loss anywhere from one millisecond to greater than five seconds, at which point the cellular system clears the call. Signal disruptions for several hundred of milliseconds can also occur due to the signalling strategy of cellular radio, such as power stepping the mobile handset at approaches or moves away from a cell site and handoff when moving from one cell boundary to another.
Though transmission impairment experienced during any point of a data call can be catastrophic, errors encountered during the initial handshaking phase have the most significant impact, for it is at this point where signalling protocol error correction and data compression standards are established for the call duration. This is compounded by the fact that much of the initial modem handshaking runs xe2x80x9cbarexe2x80x9d, (i.e., no error correction or compression is performed to insure compatibility with older land modems lacking such features). Plus, standard cellular-specific error protection techniques such as ARQ (automatic retransmission request for erroneous data) and FEC (forward error correction) introduce significant lapses in the data stream which could, at this initial stage, be misinterpreted as a carrier loss event. For example, cellular signal fade experienced in the initial connect tone exchange may cause a land modem to mischaracterize the cellular modem""s handshaking statistics, resulting in a 1200 baud connection with no error correction or compression even though both modems may possess full V.42bis and MNP5/LAPM capabilities. A less than optimal data bottleneck is thus needlessly formed, if indeed a connection is established at all.
Prior art systems have addressed this problem by the addition of specialized hardware into the cellular network or through the use of specially adapted modems at the mobile stations and fixed-ends. For example, both the proprietary ATandT/Paradyne and the MNP10 protocols provide for reliable handshaking services. However, both the cellular modem and the fixed-end modem must recognize and support these protocols; otherwise, the handshaking sequence is left unprotected. Other systems, such as Vodaphone Ltd.""s VMACS (Vodaphone Mobile Access Conversion Service) cellular data gateway used in Great Britain""s analog Total Access Communications (TACS) cellular system eliminates the need for over-the-air modem coordination and handshaking since all cellular data transmissions follow a predefined transfer rate and error protection (i.e. through well-known cyclic CLDC algorithms). Handshaking with the PSTN modem is performed exclusively by pool modems on the fixed-end side of the VMACS gateway. However, the VMACS system is disadvantageous because it requires additional system hardware (specialized CLDC modems at the mobile end base stations) and system signalling services not provided in the typical AMPS analog cellular system while limiting the mobile subscriber to only one type of cellular modem. Plus, the meager predefined data rates only permit forward transmission at 2400 baud and reverse transmission at 150 baud. With VMACS, therefore, mass data transmission simultaneously in both directions is not possible and hardware options are limited.
It is therefore desirable that a more flexible solution be found to optimize potential data throughput and reliability in a cellular environment. A variety of cellular modems and transmission protocols ranging from basic Bell 103 to V.42bis and beyond should be supported. To reduce cost and ease the equipment upgrade process, the desired solution should rely on additional firmware rather than hardware modifications whenever possible. It would also be desirable that the reliable handshaking functions be carried out in a manner transparent to the land modem yet remain compatible with non-upgraded cellular equipment. Finally, the desired techniques should be generally adaptable to a wide variety of analog cellular systems in current and projected use, including AMPS (Advanced Mobile Phone Service), TACS, JTAC (Japanese TACs), NAMPS (Narrowband AMPS) and NMT (Nordic Mobile Telephone).
The present invention includes updated firmware in both the cellular modem and the mobile handset it controls. At the fixed-end of the cellular network, which includes the base station(s) and the mobile switching center (MSC), PSTN modems are selectively interposed between the MSC and the PSTN to carry out the handshaking operations, synchronize carrier signals and establish the data connection to the land modem at the other end. Signalling extensions to the mobile/base protocol will be necessary to reliably transport handshaking information so updated firmware in the cellular fixed-end will be added. Of significance is the fact that existing signalling methodologies inherent to analog cellular will be adapted to be used to reliably transport handshaking information over the air.
According to the present invention, a mobile subscriber initiates a xe2x80x9cdataxe2x80x9d call via communications software running on a laptop or mobile computer coupled to the cellular modem. A cellular modem informs the mobile handset coupled to it that the call will be a xe2x80x9cdataxe2x80x9d call, and the handset will broadcast a data originate message to the cellular network.
When the data compatible cellular network responds to the originate message, traffic channels are allocated and connections to the PSTN are routed in a conventional manner as though it were handling a voice call. But, unlike a conventional voice call, the cellular network switches a first PSTN modem into the junction between it and the ultimate destination modem, be it located within the PSTN, a private network or even another cellular system. When relaying traffic channel tuning information back to the mobile handset/cellular modem tandem, the cellular network indicates that it is prepared to receive the handshaking information specified for the cellular modem. The mobile handset broadcasts handshaking information for the cellular modem to the cellular network encoded in a reliable data format. Preferably, this reliable format is the same used to handle conventional control data (signalling) traffic between mobiles and the network. The handshaking information will contain all parameters necessary to remotely initiate an ITU CCITT V.22 or Bell 212A modem connection including transfer mode, speed, error correction and data compression well known to those skilled in the art.
Once received, the cellular network decodes the handshaking information and translates it in a well known manner to appropriate UN/ITU modem initialization commands. The commands are then downloaded to the first PSTN modem so it can dial and establish an optimal connection with the destination modem.
When the modem connection is made, the first PSTN modem instructs the destination modem to xe2x80x9cwaitxe2x80x9d for the cellular modem to be switched in using active pause commands. Meanwhile, a second PSTN modem listens in on the handshaking negotiation to synchronize itself with the land modem and first PSTN modem. Concurrently, the cellular network relays the connection result codes to the mobile station which originated the data call, again using a reliable transfer means.
Upon receipt of the connection result codes, the mobile handset decodes the connection information and downloads it to the cellular modem. Then, it broadcasts an acknowledge signal to the cellular network and releases the traffic channels to the cellular modem. The acknowledge signal prompts the cellular network to switch in the second assigned PSTN modem so the cellular modem can be properly synchronized when the traffic channels are released. When carrier signal synchronization and data speeds are confirmed, both PSTN modems are switched out and normal data operations commence.
Land originated xe2x80x9cdataxe2x80x9d calls will be handled similarly to their conventional voice counterparts, except that: 1) reliable handshaking operations are carried out and the originating land modem is paused before the mobile is paged for the call; 2) an extended page format is used to deliver connection result information to the mobile; 3) the cellular modem synchronizes to the land modem after handshaking is passed; and (4) the mobile station must inform the network when the cellular modem enters and exits auto-answer mode. Preferably, the mobile station preregisters its handshaking information with the network to decrease latencies during the crucial modem connection phase. Also, reregistration information will be periodically resent to refresh PSTN modem assignments and mobile location.
Therefore, the present invention presents an adaptable, primarily firmware solution to reliably transport modem handshaking information over the air without specialized PSTN modem protocols or inflexible cellular data transmission standards. The only additional hardware required would be the standard UN/ITU PSTN modems controlled by the cellular fixed-end having switchable links into and out of the PSTN interface with the land modem. A digital bus interface will preferably electrically connect the cellular modem and the attached mobile handset to permit unmodulated transfer of control signals and handshaking information. However, the cellular modem may otherwise conform to any UN/ITU recognized protocol. Thus, the reliable handshaking technique of the present invention offers a cost-efficient and flexible way to reliably establish a modem connection which in turn maximizes potential data throughput through cellular radio link.